Foamed adhesive and applications thereof

ABSTRACT

A foamed adhesive finds use in the manufacture of packaging materials, and is particularly useful in the manufacture bags comprising at least one heat sealable bag ending.

FIELD OF THE INVENTION

[0001] The invention relates to improvements in packaging materials, more specifically to packaging materials comprising foamed adhesives. The adhesives find use in useful in bag closing, laminating, susceptor patch attachment and seaming applications, and are particularly well suited for use in pre-applied application including heat seal applications.

BACKGROUND OF THE INVENTION

[0002] Water-based and hot melt adhesives are commonly used to manufacture packaging materials, including bags having heat sealable seams and closures. In conventional heat sealable bag endings, adhesive is applied in a liquid or molten state and allowed to dry. Application of adhesive is generally applied by the packaging converter, thereby allowing the packager to avoid adhesive related issues such as adhesive selection, processing, trouble shooting, inventory and maintenance of adhesive application equipment. The packager then fills the package, reactivates the adhesive and seals the bag.

[0003] While aqueous liquid adhesives are safer, easier to handle and less expensive then hot melt adhesives, water-based adhesives have a number of disadvantages associated with their use. Water-based liquid adhesives require significant drying times, require long set times, and the water contained within them tends to swell the substrates surfaces that come in contact with the adhesive, leading to warpage, wrinkling, curling or bending of the substrate. Problems such as warping increases the costs associated with packaging defects due to unacceptable appearance and the like.

[0004] A need exists in the art for adhesives that are safe, effective and provide a substantial saving in terms of cost and reduction in packaging defects. The invention fulfills this need.

SUMMARY OF THE INVENTION

[0005] The invention provides a water-based adhesive that can be applied to a substrate and later reactivated in order to bond the substrate to a second substrate. The adhesive finds use in packaging applications, in particular reactivatable heat seal applications, more preferable seam and bag ending heal seal applications. The adhesive is foamed prior to application to the substrate, such as a bag ending or the like, and allowed to dry.

[0006] One aspect of the invention is directed to a packaging container having preapplied to at least a portion thereof a reactivatable adhesive, wherein the reactivatable adhesive was applied to the container in the form of a foamed water-based adhesive. In one embodiment, the packaging container is a bag, more preferable a bag having a top or bottom ending comprising the adhesive. In one particularly preferred embodiment, the packaging container is a bag adapted to package microwave popcorn.

[0007] Any water-based adhesive that can be foamed can be used in the practice of the invention as long as the dried adhesive is thermoplastic. Examples of water-based adhesives that can be used in the practice of the invention are adhesives comprising polyvinylacetate homopolymers and ethylene/vinyl acetate copolymers. The adhesives are foamed prior to application to the substrate surface. Foam levels of from about 10% to a 60% per volume, more preferably 20 to 45% by volume, even more preferable 20% to about 30% by volume are preferred.

[0008] Another aspect of the invention is directed to a method of packaging items such as food and consumer goods, and packaged items. Examples of goods that can be packaged include food, pharmaceuticals, cosmetics, breakfast cereals, beverage containers, bakery items, dry foods (e.g., dog food), produce, household products, paper products, soaps and detergents, candy, popcorn, wet food, frozen food, diapers, light bulbs and the like. A particular preferred embodiment comprises packaged microwavable popcorn wherein the heat seal is used at both ends of the bag and on the side seam. Adhesives are formulated for complete hold and fiber tear on the bottom heat seal and side seal and proper “venting” and non-destructive fiber tear on the top.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The invention involves the use of foamed water-based adhesive useful in the manufacture of packaging materials including but not limited to bags that are heat sealable, e.g., heat sealable bag closures. The term “heat seal” as this term is used and understood in the hot melt adhesive art means that a seal is made through the use of heat.

[0010] The seal will typically be made upon the application of an external source of heat. As a more specific example of heat sealing, as may be used in the practice of the invention, a first substrate having an adhesive preapplied thereon is brought in contact with a second substrate to the bonded to the first substrate. An external source of heat, and typically pressure, is then applied, whereby the adhesive is reactivated, thereby bonding the first substrate to the second substrate. Both the first and the second substrate may desirable be pre-coated with the adhesive.

[0011] Adhesives used in accordance with the invention comprise synthetic and/or natural ingredients. When modified and mechanically agitated with air, these adhesives form highly stable, consistent foams that can be applied to a substrate and dried. Following drying the dried adhesive is thermoplastic and may be reactivated by an external energy source. Typical sources of heat for reactivation of the adhesive include but are not limited to heated platens or bars, infrared, forced hot air, flame and the like. The substrates to be sealed together with the adhesive of the invention will generally be subjected to heat (140-350° F.) and pressure (40 to 100 psi) for a period of from about 0.2 to about 3.0 seconds.

[0012] The adhesives of the invention may also be formulated to reactivate upon exposure to short durations of radiant energy. In this embodiment, the aqueous adhesive to be foamed contains an energy-absorbing ingredient. Energy absorbing ingredients for use in the practice of the invention will typically have an absorption in the range of from about 400 nm to about 100,000 nM, more preferably from about 700 nm to about 10,000 nm, even more preferably from about 750 nm to about 5000 nm. Energy-absorbing ingredients include those dyes, pigments, fillers, polymers and resins or other ingredients that are capable of absorbing energy and provide an optimal balance of absorption, reflection, transmission and conduction. Pigments and dyes are particularly preferred energy absorbing ingredients, and near infrared absorbing dyes and pigments being particularly preferred. Preferred energy-absorbing ingredients for use in the practice of the invention are broad band near IR absorbers such as Epolight 1125 (Epolene, Inc), SDA6248 (H.W. Sands Corp.), SDA2072 (H.W. Sands Corp.) and carbon black. Carbon black can be purchased from Cabot under trade name of Monarch, Regal, Black Pearl, and Elftex, or Degussa (FW series), or from Columbian Chemical Company (Raven Series). Carbon black can be manufactured by different methods such as the furnace black method, the gas (channel) black method, and the lamp black method. The key parameters affecting the radian energy absorption of carbon black prepared by these various methods are average primary particle size, surface chemistry and aggregate structure. Adhesive may be coated to either or both surfaces of a substrate to be bonded. Following activation, the substrates are brought together for bonding. If the substrate is transparent or translucent to the energy used for reactivation, the adhesive formula may be sandwiched between substrates first, and then NIR energy can be applied to initiate cure.

[0013] Foamable adhesives can also be formulated for reactivation using ultrasonic energy. Preferred adhesives will reactive upon exposure to ultrasonic energy having a frequency of from about 15 kilohertz to about 40 kilohertz. Substrates to be bonded are brought together with the adhesive sandwiched there between and pressed against an ultrasonic horn with booster.

[0014] Benefits of using the foamed adhesive of the invention include improved economics and performance, such as reduced penetration into substrates, reduced moisture and faster set times. It has been discovered that adding air bubbles to the adhesive significantly reduces moisture content. Faster drying times and a substantial reduction in incidence of warping and/or wrinkling encountered in prior art methods are possible when bags are prepared in accordance with the invention.

[0015] Water-based adhesives that can be foamed and pre-applied to e.g., a bag ending, for later reactivation and closure include, but are not limited to, synthetic resins, natural rubbers, etc that are thermoplastic in the dried state.

[0016] The adhesives of the invention can be formulated as an emulsion or a solution. Adhesives comprising single homopolymer or copolymer emulsions as well as blends of two or more homopolymers or copolymers are contemplated for use in the practice of the invention.

[0017] Vinyl acetate containing emulsions include vinyl acetate homopolymers, emulsions comprising vinyl acetate copolymers such as ethylene vinyl acetate, and various mixtures thereof. Mixtures of vinyl acetate containing emulsions and vinyl acetate copolymer containing emulsions may be used. Included are ethylene vinyl acetate, copolymers of vinyl acetate and other acrylates such as 2-ethylhexylacrylate, and vinyl acetate homopolymers. Emulsions comprising a vinyl acetate polymer and/or copolymer and an acrylic copolymer may also be used in the practice of the invention.

[0018] The foamable adhesives of the invention may also comprise, in addition the vinyl acetate containing component, an acrylic copolymer containing component. Typical acrylic copolymers useful in the practice of the invention comprise acrylic monomers such as the C₁-C₁₂ esters of acrylic or methacrylic acid a well as ethylenically unsaturated monomers containing functional groups. Nonlimiting examples of acrylic copolymers include methyl methacrylate/butyl acrylate/2,2-hydoxyethylhexyl acrylate copolymers and methyl methacrylate/ethyl acrylate/2,2-hydoxyethylhexyl acrylate copolymers and the like. Both high and low molecular weight acrylic polymers are contemplated for use.

[0019] The acrylic polymer is formed of acrylate monomers. Both high and low Tg alkyl acrylates may be used. Low transition temperature monomers are those having a Tg of less than about 0° C. Included are alkyl acrylates that have up to about 18 carbon atoms in the alkyl group, preferably from about 4 to about 10 carbon atoms in the alkyl group. Such alkyl acrylates include butyl acrylate, amyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, decyl acrylate, dodecyl acrylates, isomers thereof, and combinations thereof. A preferred alkyl acrylate for use in the practice of the invention is butyl acrylate.

[0020] While the monomer system making up the acrylic copolymer may be solely based on low Tg alkyl acrylate ester monomers, inclusion of high Tg monomers and/or functional comonomers, in particular carboxy-containing functional monomers, and/or, even more preferably, hydroxy-containing functional monomers is preferred.

[0021] High Tg monomer components which may be present, and in some embodiments are preferably present, include methyl acrylate, ethyl acrylate, isobutyl methacrylate, vinyl acetate, styrene-butadiene copolymers and/or styrene-acrylate copolymers. The high Tg monomers may be present in a total amount of up to about 50% by weight, preferably from about 5 to about 50% by weight, even more preferably from about 10 to about 40% by weight, based on total weight of the hybrid polymer.

[0022] The acrylic polymer may also comprise one or more functional monomers. Preferred are carboxy and/or hydroxy functional monomers. Carboxy functional monomers may be present in the polymer in an amount of up to about 7% by weight, more typically from about 1 to about 5% by weight, based on the total weight of the monomers. Useful carboxylic acids preferably contain from about 3 to about 5 carbon atoms and include, among others, acrylic acid, methacrylic acid, itaconic acid, and the like. Acrylic acid, methacrylic acid and mixtures thereof are preferred.

[0023] The acrylic copolymer may comprises hydroxy functional monomers such as hydroxyalkyl (meth)acrylate esters, and acrylic polymers used to form the backbone of the invention are preferably acrylic ester/hydroxy (meth)alkyl ester copolymers. Specific examples of hydroxy functional monomers include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate. Hydroxy functional monomers are generally used in an amount of from about 1 to about 10%, preferably from about 3 to about 7%.

[0024] Other comonomers can be used to modify the Tg of the acrylic polymer or to further enhance adhesion to various surfaces. Such comonomers include styrene, N-vinyl pyrrolidone, N-vinyl caprolactam, N-alkyl (meth)acrylamides such as t-octyl acrylamide, cyanoethylacrylates, diacetoneacrylamide, N-vinyl acetamide, N-vinyl formamide, glycidyl methacrylate and allyl glycidyl ether.

[0025] Other components typical of adhesive compositions may be added to the foamable compositions. Said additives include, but are not limited to, plasticizers, acids, waxes, synthetic resins, tackifiers, defoamers, preservatives, bases such as sodium hydroxide, fillers, dyes, pigments, UV indicators, crosslinkers and other additives commonly used in the art.

[0026] The adhesive may also contain a surface-active agent. Examples of surface-active agents include anionic, cationic, amphoteric, or nonionic surfactants, or mixtures thereof. Suitable anionic surfactants include, alkyl sulfonates, alkylaryl sulfonates, alkyl sulfates, sulfates of hydroxylalkanols, alkyl and alkylaryl disulfonates, sulfonated fatty acids, sulfates and phosphates of polyethoxylated alkanols and alkylphenols, and esters of sulfosuccinic acid. Suitable cationic surfactants include, alkyl quaternary ammonium salts, and alkyl quaternary phosphonium salts. Suitable non-ionic surfactants include the addition products of 5 to 50 moles of ethylene oxide adducted to straight-chain and branched-chain alkanols having 6 to 22 carbon atoms, alkylphenols, higher fatty acids, higher fatty acid amines, primary or secondary higher alkyl amines, and block copolymers of propylene oxide with ethylene oxide, and mixtures thereof. When used, the surface active agent will typically be added in amounts up to about 20% by weight, based on the foamable composition as a whole. More usually from amounts of from about 0.05 to about 20% by weight, and preferably at from 0.2 to 2% by weight.

[0027] Suitable fillers are those fillers known in the art as adhesives fillers and include polysaccharides, calcium carbonate, clay, mica, nut shell flours, silica, talc and wood flour. A clay filler may typically be used to prepare adhesives for use in the practice of the invention. The addition of a filler allows for foam generated to remain consistent and stable for several hours.

[0028] Preservatives for use herein include those conventionally used in aqueous adhesives such as benzoates, amides and fluorides such as sodium fluoride. Also included are the hydroxybenzoic acid esters such as p-hydroxybenzoic acid methyl ester or p-hydroxybenzoic butyl ester. Commercially available preservatives which may be used in the practice of the invention include KATHON LXE sold by Rohm & Haas Company and Nipacide OBS sold by Clariant. The preservative will generally be included in amounts of from 0.05% to about 0.2% by weight.

[0029] Preferred adhesives are foamable adhesives comprising from about 80 to about 100% by weight of a polymer emulsion comprising a vinyl acetate homopolymer or ethylene-vinyl acetate copolymer.

[0030] The foamable adhesive composition of the invention is foamed by the addition of energy, by means known in the art such as, but not limited to, by mechanical and/or chemical means. Air or other gases are added to the foamable adhesive composition along with the addition of said energy to produce a stable, consistent foamed adhesive. Preferably air is used to produce the foamed adhesive. The adhesive foam may be produced by mechanical means such as mechanical stirring or agitation, introduction of gases or by chemical means.

[0031] The amount of air dispersed in the adhesive can vary depending on the particular formulation, but will generally be from about 10% (by volume) up to about 50% (by volume) or greater.

[0032] The foamed adhesive of the invention is particularly well suited for use in the preparation of bags having a closable end and an opposite closed or open end. Alternatively, both ends of the bag may be closable. Both flat tube style bags and bags having gussets may be manufactured in accordance with the invention. Also encompassed are block bottom and satchel bottom bags. Bags of the invention may constitute a secondary package (sealed bag within a cereal box) or be the primary package such as a popcorn bag. The containers of the invention can be used for filling, storing, shipping and cooking food, such as popcorn.

[0033] The packaging material of the invention is typically a paper product. Paper products are defined herein as any article of manufacture, at least a portion of which comprises paper. The paper product may be made totally of paper or partially of paper. The invention encompasses paper products made of either single or multiple layers, e.g., a paper laminate, plastic/paper laminate.

[0034] A preferred packaging bag useful for filling, storing, shipping and cooking popcorn. In one embodiment, the bag comprises The bag comprises front and back panels and inwardly extending first and second gusseted side panels. The front and back panels and the first and second gusseted side panels each have an exterior surface and an interior surface collectively defining an interior cooking region, a top end and a bottom end. In the preferred embodiment of the invention, each of the front and back panel means and the first and second gusseted side panel means of the bag means comprises an inner ply of substantially grease proof paper material and an outer ply of paper material.

[0035] The bottom ends will typically be operably configured to form a substantially sealed bottom region through adhesive attachment. In one such embodiment, the adhesively attached bottom ends are then further crimped and attached by adhesive to a portion of the exterior surface of one of the front and back panel means to further secure the substantially sealed bottom region.

[0036] Each of the top ends of the front and back panel means and the first and second gusseted side panel means are operably sealable to one another after filling and prior to cooking, and then releasable from each other after cooking, by bag closure means. Accordingly, the bag closure means form a substantially sealed top region after the food is operably positioned within the interior region, which may be re-opened for exposure to the food after cooking.

[0037] In the practice of the invention, the bag closure means comprises adhesive that is are operably positioned adjacent to the top end of the interior surfaces of the front and back panel means and the corresponding adjacent portions of the first and second flaps of the first and second gusseted side panel means. The bag closure means are releasable after the food has been fully cooked within the interior region of the bag means. Typically, a thin layer of a water-based foamed adhesive is applied and allowed to dry, thereby forming a thin layer of a thermoplastic adhesive. The thermoplastic adhesive is then activated by heat sealing after the bag is filled.

[0038] Bags of the invention may also comprises a heat enhancement means comprising a heat absorbing susceptor member which is operably attached to a least a portion of the front panel adjacent the food in the interior region of the bag. The heat enhancement means may be positioned between the inner and outer plies of paper material or, may be directly exposed to the food without the intervening ply.

[0039] The foamed adhesive of the invention may be used to seal, either directly or by heat seal means any side seams, the top and bottom ends, to laminate bag plies and/or for attachment of a heat enhancement means.

[0040] The adhesive formulations of the invention may be pre-applied in a continuous or discontinuous, e.g., as evenly spaced beads or dots, manner depending on surface area and coating weight desired. Particular patterns may be used to optimize substrate/adhesive contact. Depending on the adhesive, the bead size, thickness, distance apart and pattern will vary. The adhesive may be pre-applied to the substrate by any method known in the art, and include, without limitation roll coating, painting, dry-brushing, dip coating spraying, slot-coating, swirl spraying, printing (e.g., ink jet printing), flexographic, extrusion, atomized spraying, gravure (pattern wheel transfer), electrostatic, vapor deposition, fiberization and/or screen printing. The method of pre-application to the substrate is not critical to the practice of the invention.

[0041] The invention is further illustrated by the following non-limiting examples.

EXAMPLE 1

[0042] An adhesive comprising 100.0% by weight of an ethylene/vinyl acetate copolymer (81% vinyl acetate, 19% ethylene) was prepared and mechanically foamed at a 20-30% by volume foam level.

[0043] The foamed adhesive was used to coat the surface of a paper substrate and allowed to dry. A second paper substrate was brought in contact with the dried adhesive and subjected to heat (300° F.) and pressure (70 psi) for 0.4 seconds.

[0044] Heat sealability, bond strength and fiber tear was tested. 100% fiber tear was observed. Substrate coated with the dry adhesive was stored for 6 months and again tested for heat sealability, bond strength and fiber tear. Aging studies showed no loss of reactivatability.

EXAMPLE 2

[0045] An adhesive comprising 99.8% of a polyvinyl acetate homopolymer and 0.2% by weight of a defoamer combination comprising 0.1% Hodag Antifoam PX-653, supplied by Celgene Chemical Co. and 0.1% Foamaster V, supplied by Cognis Corp., was prepared and mechanically foamed at a 20-30% by volume foam level.

[0046] The foamed adhesive was used to coat the surface of a paper substrate and allowed to dry. A second paper substrate was brought in contact with the dried adhesive and subjected to heat (300° F.) and pressure (70 psi) for 0.4 seconds.

[0047] Heat sealability, bond strength and fiber tear was tested. 100% fiber tear was observed. Substrate coated with the dry adhesive was stored for 6 months and again tested for heat sealability, bond strength and fiber tear. Aging studies showed no loss of reactivatability.

EXAMPLE 3

[0048] An EVA based waterborne emulsion comprising 88 EVA Emulsion (Dur-o-set E-200, Vinamul), 7.5 Diethylene/Dipropylene Glycol Dibenzoate Plasticizer, 4 Water and 0.5 Carbon Black (Plack Pearls 4750, Cabot) was prepared by pre-dispersing carbon black in the plasticizer using a rotor-stator. The EVA emulsion and water was added using moderate speed axial paddle stirring. Following mechanically foaming of the adhesive at a 20-30% by volume foam level, the adhesive was applied and then was dried to the surface of a tack free film. The adhesive layer was later reactivated by exposure to a NIR energy source for a time sufficient to achieve melting of the adhesive material. The substrates were quickly mated together. 24 hours following bonding the substrates were pulled apart and resulting bond force and the percent fiber tear recorded. Good bond strength and 100 fiber tear is observed with both paperboard and plastic substrates.

EXAMPLE 4

[0049] An ethylene vinyl acetate based adhesive available from National Starch and Chemical Company (Product No. 40-1103) was foamed and used to precoat a substrate. This adhesive contains about 88 wt % EVA base resin, 8 wt % plasticizer, 0.2 wt % surfactant and water.

[0050] Adhesive in a bead shape form and a coat weight of 1.5 g/m was cast onto paperboard substrates 2 inches long, 1 inch wide and 2 mm thick. After 24 hours, a second substrate was placed upon the adhesive bead present on the paperboard substrate. Second substrates tested were paperboard and plastic. The substrate layers with the reactivatable adhesive layer sandwiched in the middle thereof was pressed into an ultrasonic horn with booster and ultrasonic energy 20 kHz was applied for 5 seconds. 24 hours following ultrasonic bonding the substrates were pulled apart and resulting bond force, adhesive bead flatness and the percent fiber tear recorded. Good bond strength, almost total bead flatness and 100 fiber tear is observed with both paperboard and the plastic substrates.

[0051] Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. The specific embodiments described herein are offered by way of example only, and the invention is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. 

1. An article of manufacture having preapplied to at least a portion thereof a reactivatable adhesive, wherein the reactivatable adhesive was applied to the container in the form of a foamed water-based adhesive.
 2. The article of claim 1 which is a packaging container
 3. The container of claim 2 which is a bag.
 4. The container of claim 3 comprising at least one closable bag ending, said adhesive being present on said bag ending.
 5. The container of claim 2 wherein the container is a bag adapted to package microwave popcorn.
 6. The container of claim 2 wherein the foamed adhesive comprises at least one resin emulsion.
 7. Th container of claim 2 wherein the foamed adhesive comprise polyvinyl acetate.
 8. The container of claim 2 where the adhesive is foamed to from about 20% by volume to about 60% per volume.
 9. The container of claim 4 wherein the bag ending is closed by application of heat and pressure.
 10. The bag of claim 3 wherein the bag is made of coated paper.
 11. A process of packaging an article by forming, filling and sealing a package that has had applied to at least a portion thereof a reactivatable adhesive, said reactivatable adhesive being applied as a foamed water-based adhesive.
 12. The process of claim 11 wherein the package is a bag.
 13. The process of claim 11 wherein the reactivatable adhesive is a thermoplastic adhesive.
 14. The process of claim 11 wherein the package is sealed by the application of heat and pressure.
 15. The process of claim 11 wherein the reactivatable adhesive comprises an energy absorbing ingredient.
 16. The process of claim 15 wherein the energy absorbing ingredient is a NIR absorbing ingredient.
 17. The process of claim 11 wherein the adhesive is reactivated using ultrasonic energy. 